Although functional coronary arterial circulation is vital to cardiac viability and function, very little is known about how the branching pathways of the coronary arterial network are established during embryogenesis Our retroviral cell lineage studies of the chick embryos have demonstrated conclusively that coronary progenitors arises from the epicardial anlagen and that coronary progenitors enter the tubular stage heart with the growing epicardial mantle. In addition, coronary vessels are established by vasculogenic mechanisms and not by angiogenic sprouting form the aorta. We have also shown that coronary vasculogenesis is more prominent in the outer than the inner myocardium and serves as a template for coronary arterial development. This morphogenetic process can be altered by the local expression of exogenous genes (e.g., FGF) using retroviral-mediated gene transfer. Finally, ablation of cardiac neural crests results in the loss of differentiation of the intramural, but not the subepicardial, arterial network. These data lead to the central hypothesis that paracrine signals form the myocardium regulate both the directed migration of coronary vessel precursors to the heart (Hypothesis-1) and capillary plexus formation within the heart (Hypothesis-2). We also hypothesis that a sub-population of neural crest-derived cells regulate the differentiation and patterning of the coronary arterial network (Hypothesis-3). We will test these three specific hypotheses by: 1) determining the region-specific interaction between pro-epicardial cells and myocardium-derived instructive signals and analyzing myocardial signaling properties; 2) reprogramming the gradient of the myocyte-derived FGF within the heart wall by using retroviral vectors expressing FGF-ligands and quantifying alteration of coronary patterning using lineage-specific markers; and 3) retrovirally expressing endothelin, NT3/trkC, or their dominant negative mutants to alter their expression in the developing heart and assaying neural crest migration, differentiation and apoptosis in vivo to determine their requirement for intramural arterial development. The studies proposed here will identify the regulators in establishing coronary artery network and build a foundation for rational therapeutics of coronary disorders in adults.